Field trip to Racine Reef Complex, Thornton Quarry, Illinois

Transcription

1 Field trip to Racine Reef Complex, Thornton Quarry, Illinois Primary objectives for this fieldtrip 1) Collect and identify the fossils of the Racine Reef Complex. 2) Discuss procedures for collecting fossil samples 3) Recognize common reef environments (reef core, flanking beds, and inter reef deposits) and sedimentary rocks (dolostones and dolomitic mudstones) 4) Collect and describe the fossils found in each depositional environment. 5) Describe taphonomy (preservation) of Racine fossils. 6) Describe paleoecology of Racine fossils. 7) Propose several research projects that could be conducted in the Racine Formation. Today we are visiting one of the best exposed fossils reefs known (and in one of the largest quarries ever made). The rocks at Thornton Quarry, in Thornton, IL, are Silurian in age, and are officially named the Racine Formation (Fig. 1). The main product of the quarry is dolostone aggregate (crushed rock, sand and gravel) that is used in construction, roads, and industrial uses. Stratigraphy, paleogeographic setting, and reef depositional environments During the Silurian, present day Illinois was located just south of the equator. The rocks where Thornton is located today (Fig. 2) were a giant tropical reef complex; the reef weʹre visiting is estimated as 2.7 Km long and more than 150 m thick (Mikulic and Kluessendorf 1994). The reef existed along a Fig. 1. Map of eastern Illinois showing location and age of large shallow carbonate bank located Thornton reef complex. Black areas represent the cities of Chicago, Gary, and Milwaukee. (Loydell, et al., 2002) between the deeper Illinois and Michigan basins. Index fossils of graptolites and chitinozoans (Loydell, et al. 2002) place the Racine rocks as spanning Wenlock Ludlow age. When the seas receded later in the Silurian, the restricted basins produced immense evaporate salt deposits. 1

2 Like most reefs, the Racine reef complex has a typical mounded morphology (Fig. 3), with highly tilted flanking beds. Although the stratigraphic law of original horizontality states that tilted beds were originally horizontal, that is not the case in such reef flanks, where sediments deposited along the reef core top would cascade down the sides of the reef into the deeper inter reef deposits to the sides. Sedimentologically, the core and flanking beds of the Racine Formation includes very Fig. 2. Map of Thornton Quarry. (Donovan, et al., 2002) pure dolostone (limestone altered by replacement with magnesium). Because of the extensive nature of the Racine reef complex surrounding much of the Great Lakes and its preservation as a highly resistant rock (Fig. 4), it also forms the Niagara Escarpment, from which many notable waterfalls and other Fig. 3. Typical outcrop view of reef mound stratigraphy, showing dome-shaped reef crest, vertically aligned flanking beds, and horizontal inter-reef deposits. prominent cliff faces occur. The (Modified from inter reef deposits represent slightly deeper water deposits, and include muddier dolostones. (A recently discovered Lagerstätten has been described from these inter reef deposits.) The deepest sections can include green shale. Fossils associated with high energy, shallow water reefs are commonly highly fragmented and disarticulated because of the persistent wave energy. But it is not uncommon to find original, in situ preservation of framework reef constructors (corals, stromatoporoids, etc.) (Fig. 5). Original topography of the reef core and reef mounds is also locally common. Because of Fig. 4. Silurian reef complexes in the midcontinent. 2

3 the dolomitization (replacement of fossils by the mineral dolomite), fossil preservation in such deposits is also often poor, consisting of molds and casts, recrystallized dolomite, and recrystallized pore spaces (such as the calyx of crinoids). Fig. 5. Typical Silurian reef biota. QUESTIONS TO ANSWER FOR YOUR FIELDTRIP REPORT Stratigraphy and depositiona al environments 1. What age is the Racine Formation? 2. On the next page, draw outcrop perspectives for quarry walls in the quarry. Draw the figures so that you can leave room for interpretations and location of indicative fossils and sedimentary rocks. Make sure to note the following: a) Dominant rocks (lithologies), bedding planes, and depositional environments at this site. You may be able to note dolomitized massive reef core, tilted flanking beds, and more horizontally bedded, muddy dolostone, inter reef beds. 3

4 b) Thickness of important lithologies. c) Presence of sedimentary structures (ripples, cross bedding, etc.). d) Presence of fossils, trace fossils, and taphonomic conditions. Outcrop drawing (with labeled items on prior page) of the Racine Reef complex 4

5 Paleontology and paleoecology 3. Collect fossils from all fossiliferous units. Which units are most fossiliferous? 4. Note the fauna included in the various depositional environments. Briefly note their abundance using a scale of rare (<1% of individuals), uncommon (1 5%), common (10 25%), abundant (>25%). 5. What phyla, classes, and orders dominate (are most abundant) in each depositional environment? 6. Does the fauna overall represent Cambrian, Paleozoic, or Modern evolutionary fauna? 5

6 7. What are the general ecological characteristics of these taxa? (i.e., mobile or sedentary? epifaunal or infaunal? suspension feeders, predators, algae grazers, etc.?) 8. Can you find any trace fossils? If so, what do they have to add to your interpretation of the taphonomy and paleontology of these beds? For example, were such sediments oxygenated or dysaerobic (oxygen starved)? 9. Do you see any evidence for in situ reefs, patch reefs, or brachiopod bioherms (mounds)? 10. Do you see any evidence for other biological communities? 6

7 Taphonomy 11. What is the taphonomic condition of the various reef environments? Are fossils present? If so, are they intact? In situ? Abraded? Fragmented? Articulated? Sorted according to size or shape? 12. What is the fabric of shell preservation in the various depositional environments? Are shells preferentially found in a particular orientation? Do beds represent high energy storm deposits or calmer background sedimentation? 13. What are the dominant modes of fossilization? Are there molds, casts, recrystallization, silicification? 14. Is there a bias in which fossils are preserved? For example, are there biases in shell mineralogy, mode of preservation, or ecology? 15. Is the overall taphonomy consistent with the depositional environments inferred here by the sedimentary rocks? 7

8 Group research project 16. Based on your experience with this formation today, describe two possible paleobiology projects that you could do involving this outcrop. What hypotheses would you test? Why would your research be important? How would you go about collecting relevant samples? Briefly collect some data as a pilot study to see whether your hypotheses might be accurate. Bibliography Donovan, S., R.K. Pickerill, D.G. Mikulic, and J. Kluessendorf Upright crinoids of the Thornton Reef, Wenlock (Silurian) of Illinois, USA. Geological Journal 31: Loydell, D.K., G.L. Mullins, P. Männik, D.G. Mikulic, and J. Kluessendorf Biostratigraphic dating of the Thornton Fossil Konservat Lagerstätte, Silurian, Illinois, USA. Geological Journal 37: Mikulic, D.G. and J. Kluessendorf The classic Silurian reefs of the Chicago area. GSA Northcentral Section Annual Meeting Field Trips Guidebook

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